Raster scanning is the pattern of image reconstruction used in some optical imaging systems, such as printers, projectors, and other display systems. Raster scanning is the pattern of image storage and transmission used in many bitmap imaging systems. In a raster scan, an image may be divided into a sequence of horizontal scan lines. Each scan line may be transmitted in the form of discrete pixels. When displaying an image, each pixel may be scanned in a scan line across an image plane. After each scan line, the position of the scan line may be advanced, typically downward across the image plane in a process known as vertical scanning, and a next scan line may be transmitted and displayed. This ordering of pixels by rows may be termed raster scan order and may be managed with a video controller.
Micro-mechanical devices or micro-electric mechanical systems (MEMS) are micron-scale devices, often with moving parts, and may be fabricated using traditional semiconductor processes such as optical lithography, metal sputtering, oxide deposition, and plasma etching, which have been developed for the fabrication of integrated circuits.
Micromirrors, such as the DMD™ micromirror array from Texas Instruments, are a type of micro-mechanical device. Other types of micro-mechanical devices include accelerometers, pressure and flow sensors, gears, and motors. Pivoting or oscillating torsional hinged mirrors provide very effective yet inexpensive replacements for spinning polygon shaped mirrors in printers and some types of display systems. As a MEMS mirror oscillates, the resultant reflected beam of light may be scanned onto an image plane. The scan may be a right going scan and then as the mirror changes directions the scan may be a left going scan. Further, other torsional hinged mirrors may act as position indicators, pointer mirrors, or slower speed vertical scan mirrors. Many of these MEMS devices have found wide commercial success.
In many applications, such as the example applications above, it is useful to know the phase, frequency, and/or amplitude of a MEMS mirror. If the phase of the mirror is not known and the left going scanned beam is out of phase with the right going scanned beam, a phenomenon known as “image tearing” may occur in a raster scan. FIG. 1 shows a magnified example of the phenomena. In FIG. 1, image 100 and image 110 show pixels 102 formed from a scanned beam. Each image shows left going rows 114 and right going rows 112. Image 100 shows portions of correctly scanned pixels. Each pixel 102 is in an orthogonal row and column with respect to the adjacent scanned lines. Image 110, however, shows the image tearing effect. In image 110, it can be seen that right going scan line 112 and left going scan line 114 are not in sync, therefore pixels 102 in image 110 do not line up in orthogonal rows. It is obvious that in a larger image this phenomenon may cause “ghosting” and other image distortions.
In some known systems, such as for example, a laser printer system, the needed frequency and phase information from the MEMS mirror oscillations may be provided by optical feedback. The light reflected from the MEMS mirror may be detected by sensors located at or near the photosensitive media of the printer system. The sensors in the printer system may detect the scanning beam as light from the scanning mirror impinges on the printer system sensor. A mirror driver controller may use this information in driving the scanning mirror and coordinating the vertical scan. This system lacks the flexibility a projection optical imaging system may need. It may be impractical in some optical imaging systems or other MEMS systems to place sensors in or near the image plane.
Another method may be to optically sense mirror position within the MEMS device by detecting light reflected from the backside of a MEMS mirror. This method may require additional processing, space in the system, and expense.
Yet another system may be a piezo resistance technique. Using this method, the mirror-hinge structure may have piezo resistive material implanted, or otherwise coupled, into the hinge regions of the mirror structure. The mirror structure may have metal lines that connect the sensor to instrumentation. The metal lines may traverse one or more hinges. Employing this method entails encumbering the mirror/hinge structure with sensors and metal lines. In addition, this method may incur additional processing expense and the potential for early wear out of the part.
As consumer markets drive the optical imaging systems to ever smaller and less expensive designs, a new method of mirror position feedback is needed to fit the confinements of compact MEMS mirror system design, while maintaining a low cost, robust product.